Present plastic packaging techniques involve molding a plastic package "body" around a semiconductor die. Prior to molding, the die is attached to a lead frame, or the like, having a plurality of leads. The leads have inner portions within the package body, and outer portions exiting the package body for connecting to external circuits, such as by conductors on a printed circuit board. Various forms of plastic packaged ICs are known, including DIP (Dual In-line Package), PQFP (Plastic Quad Flat Pack) and PLCC (Plastic Leaded Chip Carrier).
Microelectronics Packaging Handbook, edited by Tummala and Rymaszewski, published 1988 by Van Nostrand Reinhold, discloses transfer molding at pages 578-591. Generally, as described therein, transfer molding has been and still is the standard workhorse of the electronic packaging industry. It is an automated version of compression molding in which a preform of plastic compound is forced from a pot into a hot mold cavity. The molds are steel, and have top and bottom "halves". Each mold half has a cavity defining the size, shape and surface finish of a plastic IC package. "Gates" are small openings into the cavities where the molding compound is injected. "Vents" are other small openings allowing air to escape the cavity when molding compound is injected.
The molding compound is typically a polymer that is a solid at room temperature. It is melted prior to transfer to the cavity. The viscosity of the melted compound is generally relatively high, and the transfer occurs at elevated temperatures and pressures--typically at 900 psi (pounds per square inch) and 175 degrees centigrade.
One problem in the molding process is minimizing "wire sweep". Wire sweep refers to the displacement and/or distortion of wires attaching the semiconductor die to the lead frame (or the like) as the molding compound is injected into the cavity. As noted in Microelectronics Packaging Handbook, gates are usually placed in the bottom mold half, so that the "jet" of molding compound is directed away from the bond wires.
Another approach to minimizing wire sweep is to provide gates in both the top and bottom mold halves, as disclosed for example in copending, commonly-owned U.S. Patent Application No. 619,107, filed 11/27/90 by Schneider and Fehr.
FIGS. 1A and 1B show a typical plastic-packaged semiconductor device 100 of the prior art. A semiconductor die 102 is connected, such as by a number of bond wires 104, to inner ends of a corresponding number of leads 106. Other techniques for connecting the die to the inner ends of the leads are known. A plastic body 108 is formed about the die 102 and the inner ends of the leads, by any of a number of known molding processes. The exposed outer portions of the leads 106, exterior the body 108, are bent downwardly (indicated by the dashed line "C") and outwardly (indicated by the dashed line "D") to form what is commonly termed a "gull wing" configuration. Each lead 106 has a width (w) on the order of eight thousandths of an inch (0.2 mm), and the spacing (s) between adjacent leads 106 is typically on the order of twenty thousandths of an inch (0.5 mm).
The wires used to interconnect the die to the leadframe are typically made of pure gold having a diameter on the order of 0.0013 inch. The length of the bond wires can extend in length up to about 0.160 inch. In each device, these bond wires may be spaced apart at separations of only 0.005 inch.
FIG. 1B shows a lead form (or frame) 120 having a plurality of leads 106. The lead frame is formed from a conductive foil having a thickness (t) on the order of a few thousandths of an inch (e.g. 0.004-0.006 inch). The material for the leads 106 is typically copper, or "Alloy 42". As shown, the leads 106 terminate in an outer square ring portion 122 of the lead frame 120, from which the completed (packaged) device is ultimately excised, as indicated by the dashed line "A". Of particular note in FIG. 1B are "dambars" 124 bridging adjacent leads 106 at a position indicated by the dashed line "B" (closely adjacent or immediately exterior to the body 108. The dambars 124 are formed from the conductive material forming the leads 106, and hence are of the same thickness as the leads 106. These dambars 124 aid in maintaining alignment between the inner ends of the leads, although a die attach pad (not shown) formed from the foil is typically employed and will serve the same purpose. More importantly, however, the dambars 124 are critical in the molding process, discussed hereinbelow. (Since the leads 106 create a gap between the clamshell halves of the mold, the dambars 124 prevent plastic from "flashing" between the leads 106 exterior the body 108. After the die is packaged in the plastic body, the dambars 124 are excised, and any residual plastic flash between the outer portions of leads 106 is cleaned out in a "dejunking" step. )
FIG. 1C shows, generally, a tape-mounted semiconductor device assembly 10, as described in copending, commonly-owned U.S. Patent Application No. 454,752, entitled HEAT SINK FOR SEMICONDUCTOR DEVICE ASSEMBLY, filed Dec. 19, 1989 by Long, Schneider and Patil. The semiconductor device assembly 10 includes an upper, segmented plastic film layer 14, formed of segments 14A, 14B, 14C and 14D), a lower plastic film layer 16, metallic leads 18 sandwiched between the two plastic layers 14 and 16, a metallic (preferably copper) die attach pad 20 supported between the two plastic layers 14 and 16, a semiconductor device 22 mounted on the die attach pad 20 and bond leads 24 connecting the semiconductor device 22 to the leads 18. In lieu of employing bond wires 24, conductive bumps may be employed to provide a conductive path from the device 22 to the leads 18 in a tape automated bonding (TAB) process.
The upper plastic layer 14 does not form a continuous surface, but rather is segmented to include an inner ring portion 14A, one or more intermediate ring portions 14B and 14C disposed outside of the inner ring portion, and an exterior ring portion 14D disposed outside of the intermediate ring portions. The upper plastic layer 14 is formed of a plastic tape, such as KAPTON, and forms a thin, insulating supportive structure for the leads 18. The inside periphery of the inner ring portion 14A supports the outside periphery of the die attach pad 20, and the outside periphery of the inner ring portion 14A supports the innermost ends of the leads 18, in essence forming a "bridge" between the die attach pad and the leads. A layer-like quantity of silicone gel 28, such as Dow Corning Q1-4939, having a 1 to 10 mixing ratio of curing agent to base, encapsulates the leads 24. A body 30, formed of molding compound (described hereinafter), is formed around the device 22, leaving outer portions of the leads 18 exposed, exterior the body. The silicone gel 28 acts as a moisture barrier and a stress relief for the leads 24 during body molding, as well as prevents molding compound from contacting the semiconductor die. Surface tension between the silicone gel and the leads 24 keeps the silicone gel in place around the leads during assembly of the semiconductor device assembly. The lower plastic layer 16 covers the bottom of the die attach pad 20, and extends generally over the entire area described by the intermediate ring portion 14C, on the opposite side of the leads 18 and die attach pad 20. The lower plastic layer 16 is formed of a plastic tape material, such as KAPTON.
A "surrogate" lead frame (edge ring) 12 is provided for handling the semiconductor device assembly during manufacture thereof, and shorts the outer ends of the leads 18 to facilitate electroplating. After molding the body about the device, the semiconductor device assembly is excised from the lead frame 12 and exterior ring portion 14D, neither of which properly form any part of the ultimate semiconductor device assembly 10.
FIGS. 2A, 2B and 2C show transfer molding apparatus of the prior art. Transfer molding is an automated version of compression molding in which hot, liquid molding compound is forced from a reservoir, or pot, into mold cavities.
Molding compounds are typically resins, such as advanced B-stage compounds. In general purpose applications, wood-flour-filled phenolics, for instance, are fairly popular due to their excellent moldability and low cost. As powders and granules, they are also easily shaped into pellets by automatic preformers. The main drawback with phenolics is their limited colorability. When coloring is a major design factor, melamine, polyester, or urea are usually selected because there is a wider selection of shades and colors. For electronic packaging, the preferred resin is epoxy.
The mold set 200 has two halves, a top half 202 and a bottom half 204, each of which is provided with a recess 206 and 208, respectively. The recesses face each other when the mold is closed, forming a cavity 210 defining the size, shape and surface finish of a molded body (e.g., 108 of FIG. 1A). As shown in FIG. 2C, the mold halves close around the lead frame (e.g., 122 of FIG. 1B; or surrogate lead frame 12 of FIG. 1C), that close about open to receive lead frames and are closed (as shown), so that the semiconductor device (e.g., 102 of FIG. 1B) is contained within the cavity 210.
The bottom half 204 of the mold set is typically provided with a primary "runner" 212 receiving molten molding compound from a pot 214. One or more secondary runners 214 extend from the primary runner 212 to the cavity 210, in the bottom half 204 of the mold set. At the interface between the secondary runner 214 and the cavity 208 is a "gate" 216. "Gates" are small openings into the cavity 210 where the liquid molding compound is injected, and are normally found only in the bottom mold half ("chase") under the plane of the chip and the wires to minimize wire sweep. Typical gate dimensions are 60-100 mils wide (at the cavity interface) by 20-30 mils deep (from the secondary runner to the cavity). Air vent slots (not shown) are located opposite each gate to prevent partial fill and voids in the finished part.
In the case of a molding press provided with multiple mold sets (hence, multiple cavities), the layout of the runner system is balanced to provide for an even distribution of molding compound to each cavity. The object is to fill each cavity with compound of uniform density so that parts located next to a pressurized input (not shown) will have identical properties to those located at the other locations along the primary runner.
It has been noted that the flow of molten compound (plastic) into the bottom mold half causes not only wire sweep, but also can cause distortion of the die attach pad (e.g., 20, FIG. IC). Large die attach pads are forced upward in the process, destroying wire bonds or disorienting the die. The result is often reject parts, which represent waste and decreased throughput. Further, as lead count increases (lead pitch gets finer) each lead becomes increasingly more delicate, exacerbating the aforementioned problems.
These problems are particularly evident when the semiconductor device (die) is mounted to a tape, in what is termed a "tape automated bonding" (TAB) process (See e.g., FIG. 1C). Because the supporting TAB structure (e.g., 14, 16, 18, FIG. 1C) is flimsier than lead frame counterparts (e.g., 122, FIG. 1B), extra care in handling during the molding process is required. Proposed solutions include positioning delicate metal inserts (not shown) within the mold to aid in supporting the tape, or modifying the mold (not shown) to clamp down on only a predetermined portion of the tape. In the former, an additional time lag is introduced into the molding cycle. In the latter, accurate tape indexing and operator monitoring would be required. Another option is to use a stronger tape, such as TapePac (trademark of National Semiconductor). However, the TapePac tape has a relatively low number of leads, and presents a restrictive sourcing requirement.
U.S. Pat. No. 4,987,473 discloses a leadframe (50) wherein one series of lead tips (50a, 50c, 50e..) are bent upwards, and another series of lead tips (50b, 50d, 50f..) are bent downwards to form a leadframe with multi-tier leads, for the purpose of allowing denser packaging. This patent is cited as an example of lead frames.
U.S. Pat. No. 4,994,895 discloses a hybrid integrated circuit package. By way of example, a circuit substrate (2) is mounted to a lead frame (9) having upwardly deformed inner leads (31) and lowered stages (100), and is directed to relieving stress on the substrate.
U.S. Pat. No. 4,556,896 discloses a lead frame structure. IN FIG. 12, for example, we see two mold halves (60 and 70), and an opening (50) formed in a portion (30) of the lead frame straddling the lateral edge of the cavity (61). In other words, part of the lead frame structure is modified to act as a gate.
U.S. Pat. No. 4,788,583 discloses a semiconductor element (1) mounted on a stage (2; also known as "die attach pad"), and stage bars (3 and 4; also known as "tie bars") extending from both sides of the stage for supporting the stage during the production process. In FIG. 2A therein, the stage bars (15 and 16) are shortened, and end inside the resin package (17). A two step molding process is disclosed. As shown in FIG. 5 therein, after a first, inner package molding process, portions of the stage bars (26 and 27) in the vicinity of the frames (28 and 29) are cut off. Thereafter, an outer resin package portion (19) is formed.
U.S. Pat. No. 5,018,003 discloses a lead frame (8), wherein an outer frame portion (2) is disposed in a gate portion of mold halves for the purpose of splitting the mold compound flow evenly into the top and bottom portions of the mold. Evidently mold design would be affected by whether or not the disclosed lead frame were to be used.
U.S. Pat. No. 4,043,027 discloses a process for encapsulating electronic components, showing bottom gating.
U.S. Pat. No. 4,894,704 discloses a lead frame for resin molding. A projection or extended portion (16) is formed at the edge of the inner lead part (15) nearest to a gate part (13). A resin flow passes through the mold and collides with the projection near an inlet of the cavity. The air at corners near the cavity inlet is purged away and, consequently, pressure transfer efficiency at the corners is enhanced. Notably, the projections (16) are coplanar with the lead frame.
While the patents cited above show various modifications to a lead frame, they neither disclose nor suggest the lead frame structure of the present invention.